CLI-next

**WORK IN PROGRESS, DO NOT MERGE**

Refactor all the things to eliminate all of the painful parts of dealing with the Boxen gem, like:

* everything running every preflight check, when it's not even necessary
* the complete and utter lack of real subcommands
* no good way for people to hook their own subcommands into Boxen easily

## TODO

* a whole lot

/cc @dgoodlad @jbarnette